Direct drive amplifiers are used in a variety of applications. These include a host of applications where miniaturization is important, such as video and audio applications. The following background discussion focuses on prior art related to headphones, but the limitations described below are common to all prior art direct drive amplifiers. In particular, prior art direct drive amplifiers that operate from a single power supply require series output capacitors or other costly and space inefficient schemes.
PRIOR ART FIG. 1A illustrates a typical headphone connectivity diagram 8. The right headphone lead 12 and the left headphone lead 14 couple to the right and left headphone speakers respectively represented here by a headphone load 10 to the rest of the system. Each headphone load 10 as well as the overall system is connected to a common ground 16.
PRIOR ART FIG. 1B illustrates a prior art stereo headphones system 11 using a 3-way “jack socket” design for connecting a pair of headphones to a stereo system. As shown in FIG. 1B, the 3-way jack-socket design 11 is made of three electrically isolated portions 22, 26, and 28, dividers 24 and 29, and a body 23. The design of the 3-way jack socket allows for the use of a single jack socket 11 to connect a pair of headphones via the leads 12 and 14 and the common ground lead 16. As illustrated herein PRIOR ART FIG. 1A, the 3-way jack-socket system 11 includes the tip 22, which couples the left headphone speaker to the stereo system via the lead 12. Similarly, the middle portion 28 of the jack socket 23 couples the right headphone speaker to the stereo system via the lead 14. A rear portion 26 of the jack socket 23 connects the common return for the left and the right headphones to a common ground 16 that may be connected to the stereo system chassis to form a common ground. Dividers 24 and 29 electrically isolate from each other, the various electrically charged portions 22, 26 and 28 of the 3-way jack-socket.
Each headphone may be represented by a resistive headphone load to be driven by the incoming signals. Typical value for the resistive load of a headphone speaker is in 16 to 32 Ω (ohm) range.
PRIOR ART FIG. 2 illustrates a typical headphone driver amplifier circuit 30. The headphone driver amplifier circuit 30 includes a pair of headphone amplifiers 32 and 34, a pair of DC coupling capacitors 40 and 42, and a pair of outputs leads 12 and 14 connecting the headphone amplifiers to the headphone speakers represented by the headphone load 10.
As shown in PRIOR ART FIG. 2, the incoming (driving) signals are amplified before reaching each headphone. In the cases where the headphones are used with portable electronic devices such as portable cassette players or portable CD players, a single positive power supply such as a battery is the only source of power. In a typical portable device, headphone driver amplifiers are from a single supply (e.g. a 5 volts or 3.3 volts battery). In order to accurately reflect the incoming signals amplified by the headphone amplifiers 32 and 34, the outputs of the headphone amplifiers 32 and 34 are biased at mid-rail (VDD/2) allowing for the generation of both positive and negative going signals without clipping. As a result, the output of the amplifiers 32 and 34 are at a higher DC voltage with respect to ground.
In order to prevent high currents from flowing through the headphones and having the headphones in a continuously on state, direct current (DC) coupling capacitors such as 40 and 42 are inserted in series with the output of the amplifiers 32 and 34, in order to prevent a DC current from reaching the headphones. The DC coupling capacitors 40 and 42 act as a high pass filter preventing DC and very low frequency signals from reaching the headphones. In order to reproduce low frequency input signals into the 16–32 Ω (ohm) load of a typical headphone, the value of these DC coupling capacitors needs to be in the 100–470 μF (micro Farad) range. However, the physical size of a 100–470 μF capacitor is prohibitively large and prevents miniaturization of the headphone circuitry. The physical size and cost of these DC blocking capacitors 40 and 42 is of a greater importance in the design of portable equipment and therefore implementing an amplifier topology that either completely eliminates the DC blocking capacitors or reduces their value and size is desirable.
Returning to PRIOR ART FIG. 2, the incoming signal I is input to the two power amplifiers 32 and 34. In order to generate positive and negative going incoming signals without signal clipping, the amplifiers 32 and 34 are typically biased at mid-rail (VDD/2), and thus the positive and negative power supply terminals of the two amplifiers 32 and 34 are connected to the positive power supply VDD and ground (VSS) respectively. As a result, the outputs 36 and 38 of the input amplifier 32 and 34 need to be coupled to the left and right headphones through DC blocking capacitors 40 and 42 respectively. As previously discussed, in order to reproduce low frequencies into the typical 16 to 32 ohm headphones, the size of the DC blocking capacitors has to be in 100 to 470 μF range. The physical dimensions for these internal capacitors is very large and the size prevents the much desired miniaturization of the headphone driver amplifier circuit 30.
PRIOR ART FIG. 3 illustrates one prior art solution eliminating the need for DC coupling capacitors. A prior art driver amplifier circuit 43 includes a pair of headphone amplifiers 32 and 34 directly coupled to a headphone load 10 through a pair of leads 36 and 38, and a third amplifier 44 connected to the headphone load 10 via the lead 16. The headphone load 10 (representing the headphones) is biased between ground (GND) and the supply voltage VDD. With both headphone amplifiers biased to approximately the same DC value, very little DC current flows through the headphones, and the third amplifier sinks or sources current as necessary. Although the circuit depicted in PRIOR ART FIG. 3 eliminates the need for large DC coupling capacitors, this system has the disadvantage of having a common return 16 that must now be isolated from the equipment chassis since it has a DC voltage on it. This isolation introduces additional problems such as possible circuit damage if the electrical isolation of the common return from the rest of the system fails.
Therefore, it is desirable to provide a direct drive amplifier system that operates from a single voltage supply, and which does not require the usual large DC coupling capacitors or need the physical isolation of the common return of the amplifier.